558 THE ANATOMY OF INVERTEBRATED ANIMALS. 



Echinorhynchus, becoming closely fitted to its surface, and 

 apparently persisting throughout its entire life. The devel- 

 opment of the Echinorhynchus now approaches completion. 

 The lemnisci appear. Hooks arise on the surface of the pro- 

 boscis, not, as might be supposed, from its outer cuticle, but 

 from specially modified cells of an inner membrane. The in- 

 ternal organs begin to assume their final aspect. The ex- 

 ternal form of the adult organism is rather slowly reached, 

 and a few changes which take place after transference of the 

 Ecliiyiorhyncliiis to its final host have yet to be observed. 



The Acanthocephala undoubtedly present certain resem- 

 blances to the Nematoidea., and more particularly to the Gor- 

 diacea, but the fundamental differences in the structure of 

 the muscular and nervous system, and in that of the repro- 

 ductive organs, are so great, that it is impossible to regard 

 them as Nematoids which have undergone a retrogressive 

 metamorphosis. Tn their case, as in that of the Cestoidea 

 and that of the Bicyemida^ it is, I think, desirable to keep 

 one's mind open to the possibility that anenterous parasites 

 are not necessarily modifications of free, enterate ancestors. 



The Dicyemida. — In 1830, Krohn discovered certain cili- 

 ated filiform parasites in the renal organs of Cephalopods, 

 to which KoUiker subsequently gave the name of Dicyema. 

 Recently, these strange organisms have been made the subject 

 of renewed investigation by E. van Beneden, from whose 

 elaborate memoir ^ I take the following account of their 

 structure : 



The body of a Dicyema (Fig. 158, 1.) consists of one large, 

 cylindrical, or more or less fusiform, axial cell, which extends 

 from the slightly-enlarged head-end, by which the animal is 

 attached, to its posterior extremity, and is invested by a 

 single layer of relatively small flattened cortical cells. These 

 are arranged, like a pavement epithelium, around the axial 

 cell, their edges being juxtaposed ; they are nucleated, and 

 their free surfaces are ciliated. There is no interspace be- 

 tween, the cortical cells and the axial cell, and the organism 

 is a simple cell-aggregate, devoid of connective, muscular, or 

 nervous tissues. 



The cortical cells which invest the anterior or head-end 

 of the Dicyema have peculiar characters, and are distin- 

 guished as the polar cells. They are arranged in such a 



1 " Recherclies sur les Dicyemides." ("Bulletin de I'Acad. Eovale de Bel' 

 gique," 1876.) 



